Apparatus and method for measuring electrical resistance employing constant output voltage technique

ABSTRACT

Technique of measuring resistivity of a specimen by contacting the specimen with a pair of electrodes and passing a constant current from a constant current source through the specimen between the electrodes. The potential drop between the electrodes is detected and employed to control the output of the constant current source so as to produce a potential drop between the electrodes of a predetermined value. The measured constant current from the source which produces the predetermined potential drop is a measure of the resistivity of the specimen between the pair of electrodes.

United States Patent I [151 3,646,436

Chan et al. 1 Feb. 29, 1972 [S4] APPARATUS AND METHOD FOR I 2,854,626 9/I958 Davidson et al. ..324/64 MEASURING CT C 3,312,893 4/1967 Currin etal ..324/64 RESISTANCE EMPLOYING CONSTANT Primary Exammer-Edward E.Kubasrewlcz OUTPUT VOLTAGE TECHNIQUE Attorney-Irving M. Kriegsman [72]Inventors: Joseph Y. Chan, Chelmsford; Dinesh C.

Gupta, Cambridge, both of Mass. [5 71 ABSTRACT Technique of measuringresistivity of a specimen by contact- [73] Asslgnee' GTE Laboratoriesmm-pound ing the specimen with a pair of electrodes and passing a con-[22] Filed: Dec. 22, 1969 stant current from a constant current sourcethrough the [2]] pp No 886 974 specimen between the electrodes. Thepotential drop between the electrodes is detected and employed tocontrol the output of the constant current source so as to produce apotential [52] US. Cl. ..324/64, 323/22, 324/62 R rop between theelectrodes of a predetermined value. The [5 1] Int. Cl measured constantcurrent from the source which produces [58] Field of Search ,,324/62,64, 65; 323/1, 9, 22 T the predetermined potential drop is a measure ofthe resistivity of the specimen between the pair of electrodes. [56]References (med 4 Claims, 4 Drawing Figures UNITED STATES PATENTSPATENTEUFEBZQ'IHIZ 3.64 436 2o CONSTANT CURRENT AMME'TER POWER SOURCE 12 11 I 0 WIDE ERROR I BAND DETECTION CONTROL Fig. 2A

INVIL'NI'ORS JOSEPH Y. CHAN DINESH C. GUPTA Fig.2. BY 19 21 A4,

AGENT APPARATUS AND METHOD FOR MEASURING ELECTRICAL RESISTANCE EMPLOYINGCONSTANT OUTPUT VOLTAGE TECHNIQUE BACKGROUND OF THE INVENTION Thisinvention relates to apparatus and methods for measuring electricalresistance. More particularly, it is concerned with apparatus andmethods for measuring the electrical resistivity of semiconductormaterials.

Various techniques have been developed for measuring the resistivity ofsemiconductor materials. One technique which is nondestructive anduseful over a wide range of resistivities is the spreading resistanceprobe technique. In this technique as commonly employed a specimen ofsemiconductor material is contacted by two probes or electrodes. Aconstant current is passed through the specimen between the electrodesand the voltage drop is measured. From this measurement the resistivityof the region of the specimen adjacent the probes may be determined.

Various factors influence the accuracy to which resistivity can bedetermined by the aforementioned technique. Certain of these factorsinvolve the mechanical aspects of making contact between the electrodesand the specimen and are discussed in detail in application Ser. No.886,908, filed concurrently herewith by Joseph Y. Chan and Dinesh C.Gupta, entitled Apparatus for Measuring Electrical Characteristics of aSpecimen and assigned to the assignee of the present invention. Inaddition, measurements are affected by temperature changes at theregions of contact and within the specimen due to Joule heating causedby the flow of test current.

SUMMARY OF THE INVENTION In measuring the electrical resistance of aspecimen in accordance with the present invention the specimen iscontacted by a pair of electrodes. A constant current is passed throughthe specimen between the electrodes from a source of constant currentconnected to the electrodes. A control means is connected to theelectrodes and to the source of constant current for changing the valuesof the constant current to obtain a predetermined potential drop betweenthe electrodes. When the potential drop between the electrodes is equalto the predetermined potential drop, a current-measuring means measuresthe current flowing from the source of constant current. The measuredconstant current which produces the predetermined potential drop is ameasure of the resistivity of the specimen between the pair ofelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects, features, andadvantages, of the apparatus and method for measuring resistance inaccordance with the invention will be apparent from the followingdetailed discussion together with the accompanying drawings wherein:

FIG. 1 is a block diagram of apparatus in accordance with the invention;

FIG. 2 is a perspective view illustrating certain elements of theapparatus of the invention;

FIG. 2A is a detailed view in perspective of a portion of the apparatusillustrated in FIG. 2; and

FIG. 3 is a detailed circuit schematic diagram of portions of theapparatus shown in the block diagram of FIG. 1.

DESCRIPTION OF THE INVENTION FIG. 1 is a schematic representation inblock diagram form of apparatus in accordance with the invention formeasuring the resistivity of a specimen 10. The apparatus includes twoprobes or electrodes 11 and 12 which are mounted at a fixed distancefrom each other in a measuring head 13 as illustrated in FIGS. 2 and 2A.As shown in FIG. 2 the specimen 10 is placed on a support 14 beneath themeasuring head 13. The measuring head is fixed to a mounting structure15 which is appropriately raised and lowered (by means not shown) tobring the electrodes into contact with the specimen. An improvedapparatus for raising and lowering the measuring head is disclosed indetail in the aforementioned application filed concurrently herewithentitled Apparatus for Measuring Electrical Characteristics of aSpecimen."

As illustrated in the block diagram of FIG. 1 the electrodes areconnected to a constant current power source 20. One of the electrodes11 is connected through an ammeter 21 and the second electrode 12 isconnected directly to the source 20. The first electrode 11 is alsoconnected, at a point between the ammeter 21 and the constant currentsource 20, to a control arrangement 22 which controls the output currentof the constant current power source 20. The control arrangementincludes a wide band amplifier 23, an error detection circuit 24, and acontrol amplifier 25.

The apparatus as illustrated in FIG. 1 operates generally in thefollowing manner. The electrodes 11 and 12 are brought into contact withthe specimen 10. Current from the constant current power source 20 flowsto the electrodes and through the specimen 10 between the tips of theelectrodes. The wide band amplifier 23 senses the potential on the firstelectrode 11. (The second electrode 12 is connected directly to groundwithin the constant current power source.)

The detected potential level is amplified by the wide band amplifier 23and applied to the error detection circuit 24. The error detectioncircuit 24 produces an output signal which is indicative of thedifference between the level of the applied signal from the wide bandamplifier 23 and a predetermined voltage level. The output signal isapplied to the control amplifier 25 which changes the input signal tothe constant current power source 20. The change in the input signal tothe constant current power source 20 causes a change in the value of theoutput current of the constant current power source 20 which in turnchanges the potential drop between the electrodes, as detected by thecontrol arrangement 22, to a predetermined value.

When the detected potential is at the desired value, the error detectioncircuit 24 detects no difference between its input voltage level and thepredetermined voltage level. Thus, the operation of the controlamplifier 25 and consequently the operation of the constant currentpower source 20 are stabilized. The ammeter 21 may then be read todetermine the value of constant current from the source 20 passingthrough the electrodes and the portion of the specimen between theelectrodes. The ammeter reading is a measure of the resistivity of theregion of the specimen adjacent the electrodes.

In a specific embodiment of the apparatus according to the inventionelectrodes 11 and 12 of tungsten carbide, each having a tip radius of0.0016 inch, were mounted in a measuring head 13 as shown in FIGS. 2 and2A so as to produce a constant force of contact of about 15 grams on aspecimen. The distance between the electrode tips was 0.l50 inch. Thesecond of the electrodes 12 was connected directly to the constantcurrent power source 20. The constant current power source 20 was aModel C6I2A constant current power supply produced by ElectronicMeasurements Co. Inc., Eaton-town, NJ.

The first electrode 11 was connected through an ammeter 21 to thepositive terminal of the power supply 20. The second electrode 12 wasconnected to ground and directly to the negative terminal of the powersupply 20. The positive terminal of the power supply was also connectedto the input terminal 30 of the wide band amplifier 23 as illustrated inFIG. 3. The output terminal 31 of the control amplifier 25 illustratedin FIG. 3 was connected directly to the remote terminal of the powersupply.

02 forming a differential amplifier in the error detection circuit 24.The output of the differential amplifier was passed through anemitter-follower transistor O3 to the control amplifier 25.

The control amplifier 25 included a series arrangement of a field effecttype transistor Q4 and two resistors R20 and R21. A resistor R22 of highresistance value was connected in parallel with the series arrangementand between the remote terminal of the power supply and ground. Errorsignals from the error detection circuit 24 applied to the gate of thefield effect transistor Q4 changed the impedance of the field effecttransistor Q4 and, therefore, the current flowing therethrough. Theconstant current power source 20 employed differential amplifiercircuitry and the value of the constant output current from the constantcurrent power source was regulated by the conditions produced at itsinput by the control amplifier 25.

For the specific embodiment described above, 4 millivolts was selectedas the value of the predetermined potential drop between the electrodes.This value was found to be particularly advantageous for use inmeasuring semiconductor materials within the range of resistivities from10" to 10'' ohm-centimeters. The constant current power source 20 wascapable of providing a constant current output from 1 microampere to 100milliamperes.

The values of the components in the control arrangement 22 of thespecific embodiment including the wide band amplifier 23, the errordetection circuit 24, and the control amplifier 25 are listed below.

Ql PNP transistor ZNZBOO RH 470 ohms Q2 PNP transistor 2N2800 Rl2 750ohms Q3 PNP transistor 2N2800 R l 3 820 ohms ()4 N channel field-effectR14 10 kilohms transistor -Texas Instrument type S34 Dl lN2069 RlS 200ohms potentiometer R1 330 ohms R16 500 ohms R2 10 kilohms RH 4700 ohmsR3 l kilohm Rl8 220 kilohms R4 l kilohm R19 4700 ohms R5 32.4 kilohmsR20 lOO ohms R6 820 ohms R2] 1 kilohm R7 lSOO ohms R22 megohms R8 1500ohms Cl 25 microt'arads R9 330 ohms C2 3.3 microl'arads R I 330 ohms 8--l volts ln practicing the invention as described for measuringelectrical resistance, the potential drop between the electrodes is thesamefor each measurement. That is, the IR drop does not vary fromspecimen to specimen or from measurement to measurement. In addition,the Joule heating decreases for higher values of resistance. By thechoice of a suitable value of potential drop (4 millivolts in thespecific embodiment disclosed) the IR drop will always be low. Sincethese conditions are the same for each measurement, a high degree ofreproducibility and accuracy of measured resistivities is obtained.

While there has been shown and described what is considered a preferredembodiment of the present invention, it will be obvious to those skilledin the art that various changes and modifications may be made thereinwithout departing from the invention as defined by the appended claims.

What is claimed is:

1. Apparatus for measuring the electrical resistance of a specimencomprising a pair of electrodes for contacting the specimen;

a source of constant current connected to the pair of elec-' detectedpotential drop between the electrodes, and regulating means connected tothe comparison means and to said source of constant current and operablein response to the output signal from the comparison means to vary thevalue of the constant current produced by the source of constant currentin a manner to change the value of the potential drop between theelectrodes toward the predetermined potential; and current-measuringmeans for measuring the current flow from the source of constant currentwhen the potential drop between the electrodes equals said predetenninedpotential. 2. Apparatus for measuring the electrical resistance of aspecimen in accordance with claim 1 wherein the value of the constantcurrent produced by the source of constant current varies in response tothe impedance present at the input thereto; said regulating meansincludes a variable impedance element connected to the input of thesource of constant current; and the value of the impedance of saidvariable impedance element varies in response to the output signal fromthe comparison means. 3. Apparatus for measuring the electricalresistance of a specimen in accordance with claim 2 wherein saidcomparison means includes a differential amplifier circuit and isoperable to produce an output signal proportional to the differencebetween said predetermined potential and the detected potential dropbetween the electrodes; and said variable impedance element is afield-effect type transistor having its gate connected to saidcomparison means and its source and drain coupled to the input of thesource of constant current and is operable to provide a value ofimpedance between the source and drain related to the output signal fromthe comparison means. 4. The method of measuring the electricalresistance of a specimen including the steps of contacting the specimenwith a pair of electrodes; passing a constant electrical current throughthe specimen between the electrodes; measuring the potential dropbetween the electrodes; comparing the measured potential drop betweenthe electrodes with a predetermined potential; changing the value of theconstant current to change the potential drop between the electrodes tothe predetermined potential; and measuring the electrical current whenthe potential drop between the electrodes equals said predeterminedpotential.

1. Apparatus for measuring the electrical resistance of a specimencomprising a pair of electrodes for contacting the specimen; a source ofconstant current connected to the pair of electrodes; control meansincluding sensing means connected to the electrodes for detecting thepotential drop therebetween, comparison means connected to the sensingmeans and operable to produce an output signal indicative of thedifference between a predetermined potential and the detected potentialdrop between the electrodes, and regulating means connected to thecomparison means and to said source of constant current and operable inresponse to the output signal from the comparison means to vary thevalue of the constant current produced by the source of constant currentin a manner to change the value of the potential drop between theelectrodes toward the predetermined potential; and current-measuringmeans for measuring the current flow from the source of constant currentwhen the potential drop between the electrodes equals said predeterminedpotential.
 2. Apparatus for measuring the electrical resistance of aspecimen in accordance with claim 1 wherein the value of the constantcurrent produced by the source of constant current varies in response tothe impedance present at the input thereto; said regulating meansincludes a variable impedance element connected to the input of thesource of constant current; and the value of the impedance of saidvariable impedance element varies in response to the output signal fromthe comparison means.
 3. Apparatus for measuring the electricalresistance of a specimen in accordance with claim 2 wherein saidcomparison means includes a differential amplifier circuit and isoperable to produce an output signal proportional to the differencebetween said predetermined potential and the detected potential dropbetween the electrodes; and said variable impedance element is afield-effect type transistor having its gate connected to saidcomparison means and its source and drain coupled to the input of thesource of constant current and is operable to provide a value ofimpedance between the source and drain related to the output signal fromthe comparison means.
 4. The method of measuring the electricalresistance of a specimen including the steps of contacting the specimenwith a pair of electrodes; passing a constant electrical current throughthe specimen between the electrodes; measuring the potential dropbetween the electrodes; comparing the measured potential drop betweenthe electrodes with a predetermined potential; changing the value of theconstant current to change the potential drop between the electrodes tothe predetermined potential; and measuring the electrical current whenthe potential drop between the electrodes equals said predeterminedpotential.